Minor UX cleanups for performance dashboard

Split out from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12616#note_34153096.

From the latest mockup, shown in https://gitlab.com/gitlab-org/gitlab-ce/issues/30369, there are various discrepancies. Here are some of them:

• X-axis: missing ticks
• X-axis: the space around the label should be the same as the Y-axis label
• Y-axis: horizontal lines behind the chart should be lighter colored
• Both axis: the text should have a lighter gray color
• Legend: increase the space between it and the chart
• Legend: reduce size of the color block
• Tooltip: increase padding (and maybe reduce the font size — I'm not sure about this because I'm not aware of the scale of the screenshot)